Do 125% Continuous load rules apply to sizing panels and transformers?

[JWalker23]

I have an NEC question that I cannot find a specific answer to:

Every panel schedule I have ever seen applies 125% to continuous loads to calculate the load for the panel. This calculation is typically used to size everything from the conductors, to OCPD, to panels and transformers.

There are clear NEC sections about applying the 125% for continuous loads when sizing feeders, branch circuits, and OCPD. But I can't find anything in the NEC about applying 125% as a demand factor in load calcs for sizing equipment like panels, transformers, etc.

This has recently become a question due to sizing step down transformers and panels feeding large electric vehicle charging installations. 625.42 says that we have to treat EV as continuous load. But does the 125% continuous load requirement apply to sizing transformers and panels?

Any code section references you can provide would be helpful.

Thank you.

 

[don_resqcapt19]

There is nothing in the code that I know that covers the sizing of a transformer.

The bus in a panel can be seen as a feeder, so some apply the feeder rule.

 

[wwhitney]

Panel busbars are feeder conductors, so the rules on 125% sizing for feeders (215.2 and 215.3) would apply to them.

The rating on a transformer, as I understand it, is already a continuous rating, so there is no need to apply a 125% factor to the continuous loads for transformer sizing. I believe that is also one of the reasons that the primary side protection for a transformer may be sized at 125% of the transformer rating, to accommodate a transformer serving continuous loads up to its full rating, and the necessary OCPD size on the primary side, assuming non-100% rated OCPD.

Cheers, Wayne

P.S. FWIW, Article 625 does not actually require you to treat the EVSE as a continuous load for all cases--625.42 just says "fort the purposes of this article." 625.41 does specify the 125% factor for OCPD for all cases (without the usual exception for 100% rated OCPD, which I believe to be an oversight and is the subject of a PI I have made). And that will necessitate the corresponding upsizing of feeder conductors so as still to be protected by the OCPD. But it doesn't preclude the use of 240.4(B), unlike the language in 215.2 for continuous loads.

For example, if a feeder supplies (15) 40A (continuous) EVSEs, that's a load of 600A, and 625.41 requires a minimum 750A OCPD (next size up is 800A). If that 600A is a continuous load, 215.2 requires conductors of 750A ampacity. While if if that 600A is not continuous, 240.4(B) would allow conductors of 701A ampacity.

Of course, for any single EVSE, it is clear that it is a continuous load per the definition--there are EVs that can plug it into and draw the full EVSE rating for 3 hours. Conversely, for a feeder supplying 10 EVSEs it's pretty unlikely (although not implausible) that 10 such EVs plug in at the same time.

 

[JWalker23]

Thank you gentlemen.

 

[electrofelon]

Panel busbars are feeder conductors, so the rules on 125% sizing for feeders (215.2 and 215.3) would apply to them.

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Wayne, I do not agree with that assessment. My take is that Article 310 specifically says that it does not cover conductors that are in integral part of equipment. Now 215 doesn't say that so I guess it could be argued that article 215 covers article 310 feeders AND feeders integral to equipment, but I'm not sure I subscribe to that. (And yes I am aware article 368 says "busways used as feeders" but that is a very special case of a piece equipment that is specifically for that very purpose.)

 

[wwhitney]

Wayne, I do not agree with that assessment. My take is that Article 310 specifically says that it does not cover conductors that are in integral part of equipment. Now 215 doesn't say that so I guess it could be argued that article 215 covers article 310 feeders AND feeders integral to equipment, but I'm not sure I subscribe to that.

Your comment about Article 310's scope just means that, for example, you can't use it to determine the ampacity of the busbars. I would agree that as Article 215 doesn't have the equivalent limitation, 215.2 applies to non-wire type conductors.

But I think this particular point ends up not mattering for the OP.. 215.3 tells you what OCPD size to use on your feeders. And 408.36 tells you that a panelboard is required to be protected at its rating. So your panelboard rating will need to be at least the 215.3 minimum OCPD size, and that include a 125% factor for continuous loads (absent 100% rated OCPD).

Cheers, Wayne

 

[electrofelon]

Your comment about Article 310's scope just means that, for example, you can't use it to determine the ampacity of the busbars. I would agree that as Article 215 doesn't have the equivalent limitation, 215.2 applies to non-wire type conductors.

But I think this particular point ends up not mattering for the OP.. 215.3 tells you what OCPD size to use on your feeders. And 408.36 tells you that a panelboard is required to be protected at its rating. So your panelboard rating will need to be at least the 215.3 minimum OCPD size, and that include a 125% factor for continuous loads (absent 100% rated OCPD).

Cheers, Wayne

My belief is that it is not the intent of the NEC to have something like article 215 cover integral components of equipment, but I see how it can be interpreted the other way. FWIW, 215.1 uses the word "conductors" , and I believe that is referring to article 310 conductors.

Yes I was also going to say it's somewhat of an academic point unless you get into 100% rated breakers serving the panel board.

 

[wwhitney]

My belief is that it is not the intent of the NEC to have something like article 215 cover integral components of equipment, but I see how it can be interpreted the other way.

Yes, given that the definition of feeder is "all circuit conductors," and busbars both conduct and are part of the circuit, I would say they clearly meet the definition. So as Article 215 doesn't have the "equipment" exclusion in its scoping, like Article 310 does, then Article 215 does cover panel busbars.

Cheers, Wayne

 

[electrofelon]

Yes, given that the definition of feeder is "all circuit conductors," and busbars both conduct and are part of the circuit, I would say they clearly meet the definition. So as Article 215 doesn't have the "equipment" exclusion in its scoping, like Article 310 does, then Article 215 does cover panel busbars.

Cheers, Wayne

Clearly I don't agree. They are talking about article 310 conductors . Why would they talk about conductors in factory built listed equipment?

 

[wwhitney]

Why would they talk about conductors in factory built listed equipment?

Say you have a bussed gutter (not sure what article covers that), or even an MLO panelboard with no OCPD in it (just feedthru lugs, possibly multiple sets of the plug on variety), so it's not a "panelboard" per the definition. What code section tells you how to size those busbars, if not 215.2?

Cheers, Wayne

 

[electrofelon]

Say you have a bussed gutter (not sure what article covers that), or even an MLO panelboard with no OCPD in it (just feedthru lugs, possibly multiple sets of the plug on variety), so it's not a "panelboard" per the definition. What code section tells you how to size those busbars, if not 215.2?

Cheers, Wayne

You just use the rating provided by the manufacturer. Why would you use any other value?

 

[wwhitney]

You just use the rating provided by the manufacturer. Why would you use any other value?

OK, but what section in the NEC tells you what rating is required? If you think the generic term "conductors" doesn't cover those busbars, what section uses other language?

Cheers, Wayne

 

[electrofelon]

OK, but what section in the NEC tells you what rating is required? If you think the generic term "conductors" doesn't cover those busbars, what section uses other language?

Cheers, Wayne

I think you are overthinking this Wayne. Let me turn it around and say why do you feel the need to dig thru the NEC and find some other rating value in the NEC to use or modify the rating rather than the value provided by the manufacturer of a piece of listed equipment built to a product standard?

 

[electrofelon]

BTW, bussed gutters seem to be typically listed to UL 1773, "termination boxes".

 

[wwhitney]

I think you are overthinking this Wayne.

I certainly do that, but in this case you are missing my question. Let me rephrase:

Say I have a 200A non-continuous load, and I have a 200A feeder, and I decide to put a 100A bussed gutter in the middle of the 200A feeder for no good reason. What section does this violate? What section tells us the bussed gutter has to be at least 200A in rating?

I'd say it's 215.2. But you are proposing the interpretation that the general term "conductors" doesn't cover these busbars. So what other section is there that rules out that install?

Cheers, Wayne

 

[electrofelon]

I certainly do that, but in this case you are missing my question. Let me rephrase:

Say I have a 200A non-continuous load, and I have a 200A feeder, and I decide to put a 100A bussed gutter in the middle of the 200A feeder for no good reason. What section does this violate? What section tells us the bussed gutter has to be at least 200A in rating?

I'd say it's 215.2. But you are proposing the interpretation that the general term "conductors" doesn't cover these busbars. So what other section is there that rules out that install?

Cheers, Wayne

I would say only 110.3 covers it. Transformers are the same way, nothing in the NEC specifically says you can't overload a transformer, the best you can do is 110.3 (granted the 450.3 protection requirements will indirectly limit the degree to which it can be overloaded). And same with meter sockets, I don't think anything in 312 says they must be used at or below their rating.

 

[wwhitney]

I would say only 110.3 covers it.

OK. So hopefully the bussed gutter instructions say something like "do not load in excess of their rating." Which they probably do.

Anyway, for the reasons discussed, I'll stick with the notion that 215.2 applies.

Cheers, Wayne

 

[electrofelon]

Say I have a 200A non-continuous load, and I have a 200A feeder, and I decide to put a 100A bussed gutter in the middle of the 200A feeder for no good reason.

Haha that cracked me up. I would have tried to come up with some practical application. How about I decide to use a bunch of MLO panelboards with feed thru lugs end to end to build my own busways due to supply chain issues .

Btw it is interesting the definition of panelboard does seem to require it to have a OCPD. I know Charlie's rule and all that says what it says, but I would say that's just a slightly flawed definition and a MLO panel board doesn't become a panel board only once you put a breaker in it.

 

[wwhitney]

Btw it is interesting the definition of panelboard does seem to require it to have a OCPD. I know Charlie's rule and all that says what it says, but I would say that's just a slightly flawed definition and a MLO panel board doesn't become a panel board only once you put a breaker in it.

If it has no OCPD, how it is functionally different from a bussed gutter?

Obviously it is listed to a different standard and may have different instructions from the manufacturer.

Cheers, Wayne